Design Methods - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 340

The numerical value of RMR can take on values from 0 to 100. By means of the
RMR the rock mass is subdivided into five classes ranging from “very good” to
“very poor” (Table 12.3).

Rock mass index (RMi)
The rock mass index (RMi) was introduced by Palmström (1995) and later modified
(Palmström 2000). RMi is equal to the unconfined compressive strength of intact
rock σ cIR reduced by a factor J P that reflects the influence of discontinuities. J P com-

bines the ratings of roughness, fillings, degree of weathering, persistence, spacing and
number of sets. The procedure for the determination of RMi is illustrated in Fig. 12.6.

Figure 12.6 Determination of RMi (Palmström 1995)

12.5 Flaws and Deficiencies of Classification Systems

In the following, some striking deficiencies of classification systems which can lead to
an unsafe design are demonstrated using the example of the Q system.
RQD, which is an input parameter of the Q system, is dependent on a number of influ-
encing factors such as the drilling method, the core quality, the core diameter and the
method of exploration.
As an example, Fig. 12.7 shows the result of an optical scan of a 1 m long borehole
wall (left) compared with a photograph of a conventional core of the same rock (right)
obtained from the same depth (Sommer 2009). The borehole scan reveals two discon-
tinuities with spacing of more than 10 cm resulting in a RQD value of 100%. The con-
ventional core, however, due to the drilling process, is separated into pieces < 10 cm
corresponding to a RQD value of 0.
Figure 12.8 shows a rock mass with an orthogonal discontinuity system consisting of
horizontal bedding-parallel discontinuities and vertical joints. The mean RQD evalu-
ated along three drillings, namely a vertical borehole, a borehole inclined at 45° and a